This invention relates to a process of rotationally molding and the resulting product or article in which a blend of high density polyethylene or ethylene-alkene copolymer and an ethylene-propylene copolymer or terpolymer with diene or a mixture of these two with an EVA copolymer is used.
The most pertinent prior art of which applicants are aware are the following:
U.S. Pat. No. 3,647,922 discloses polyethylene blended with an ethylene-propylene copolymer. However, the blends of this invention and the structures produced therefrom from advantages that are not achieved with the blends of this patent. Thus in this patent the copolymers are block copolymers while the ones here are essentially random as produced via transition metal catalysts. Also the ethylene content of the reference copolymer is low while in the present invention it is high. Additionally, the blends of the reference have low amounts of polyethylene while the blends of this invention have high amounts of polyethylene.
U.S. Pat. No. 3,261,889 discloses a blend of a low density polyethylene and an ethylene-propylene-diene terpolymer to improve environmental stress crack resistance (ESCR). (A table of abbreviations is presented at the end of this specification.) The three component blends of this invention especially have the ESCR improvement significantly above that achieved via a two component blend due to the unexpected synergism of the blend components chosen.
In the conventional rotational molding methods using resins and blends it is customary to use polyethylene of narrow molecular weight distribution and with low melt flow to achieve high impact strength. These prior resins are difficult to process and it appears to be due to their high apparent melt viscosity and they are generally not warp resistant. It has been known to blend EVA (ethylene-vinyl acetate copolymer) with polyethylene in order to improve the impact strength of the resin. However, polyethylene-EVA copolymer blends are not nearly as effective as ethylene-propylene copolymer or ethylene-propylene-diene terpolymer in improving the impact resistance of rotomolded structures. In addition, these prior and conventional rotational molding blends containing polyethylene do not have the above advantages of the blends of this invention.